Metastable (Atom) De-excitation Spectroscopy (MDS) provides a powerful technique with which to investigate surface electronic structure with extreme surface specificity. In this technique a thermal energy beam of noble-gas metastable atoms is directed at the surface under study and the kinetic energy distribution of ejected electrons that result from metastable atom de-excitation is measured. Although the measured distribution contains information about the electronic structure of the outermost surface layer, its detailed analysis requires knowledge of the dynamics of the metastable atom-surface interaction. In the present work, these dynamics have been investigated directly by use of spin-labeling techniques. The electron spins of the incident metastable atoms are polarized and the spin-polarization of the ejected electrons is measured with a Mott polarimeter. Energy resolve electron spin-polarization measurements are reported for a variety of sub-monolayer coverages of cesium on a Cu(100) surface, and for oxygen and cesium co-adsorption on a Cu(100) surface. The Cs/Cu(100) system exhibits large ($\sim$2.8 eV) change in the surface work function. The results of the current work suggest that MDS interactions in both high and low work function regimes are more complex than has previously been supposed. Several additional interactions are suggested to explain the data acquired. The question of the occupancy of the adsorbed cesium valence level at various coverages is also addressed.